1. Field of the Invention
The present invention relates to a carrier regeneration circuit used in a communications system that performs data transmission involving demodulating quadrature amplitude-modulated waves.
In demodulators used in such communications systems, the received signal is demodulated, to recover digital data, using a regenerated carrier synchronized to the phase of the carrier contained in the received signal. When a momentary break occurs in a radio communication line, for example, the carriers are thrown out of synchronization between the transmitting and receiving ends. Furthermore, a carrier frequency difference occurs between the transmitter and receiver. When the line is restored from the momentary break, the regenerated carrier must be instantly brought back into synchronization. The need therefore arises for a carrier regeneration circuit that has a wide capture range and that achieves quick synchronization.
2. Description of the Related Art
FIG. 1 is a block diagram showing an example of a receiver circuit at the receiving end in a digital multiplexing radio communication system, according to the prior art. Mixers 10 and 12 and a 90.degree. hybrid 14 constitute a known quadrature demodulator in which an IF signal is quadrature-demodulated by the output of a voltage-controlled oscillator 16, thereby recovering baseband signals consisting of an I-phase signal and a Q-phase signal. The I-phase and Q-phase signals are respectively amplified by amplifiers 18 and 20 to a constant amplitude, and discriminated by A/D converters 22 and 24. A phase comparator 26 outputs two-bit phase error signals so that the positions of the baseband signals are brought into coincidence with prescribed signal points (in 64-QAM, they are arranged as 64 lattice points, 8 points horizontally and 8 points vertically) in a phase plane with the amplitude of the I-phase signal plotted along the horizonal axis and the amplitude of the Q-phase signal along the vertical axis. The two-bit phase error signals output from the phase comparator 26 are input to a differential amplifier 28 which obtains the difference between them and outputs a signal of one of three levels, +, -, or 0. This signal is low-pass filtered through a loop filter 38 and then input as a control signal to the voltage-controlled oscillator 16. A sweeper 40 will be described later.
FIG. 2 shows an example of the configuration of the phase comparator 26. As shown in FIG. 2, the phase comparator 26 consists of two EOR circuits 42 and 44. The most significant bit (MSB) of the Q-phase signal and the high-order error bit of the I-phase signal are input to the EOR circuit 42, while the MSB of the I-phase signal and the high-order error bit of the Q-phase signal are input to the other EOR circuit 44. The outputs of the EOR circuits 42 and 44 are coupled to the two inputs of the differential amplifier 28 whose output, therefore, is "0" when the outputs of the EOR circuits 42 and 44 are (1, 1) or (0, 0), "-" when (1, 0), and "+" when (0, 1).
FIG. 3 shows, by taking 16-QAM as an example, the 16 signal points (marked x) in the phase plane and the range of the baseband signal value when the output of the differential amplifier 28 is +, -, and 0. For example, when the baseband signal value is at point A in FIG. 3, the I-phase and Q-phase signals are both positive, so that their MSBs are both "1". Further, since point A is to the right of signal point B and to the left of the midpoint between signal points B and C, the high-order error bit of the I-phase signal is "1". Moreover, since point A is below signal point B and above the midpoint between signal points B and D, the high-order error bit of the Q-phase signal is "0". As a result, the output of the EOR circuit 42 is "0" and the output of the EOR circuit 44 is "1", so that the output of the differential amplifier 28 is "+". Therefore, when the baseband signal is at point A in the phase plane, a control signal is supplied to the voltage-controlled oscillator 16 (FIG. 1) to rotate the baseband signal in the counterclockwise direction (+ direction) closer to point B.
FIG. 4 shows the output voltage of the differential amplifier 28 as a function of the phase difference between the local oscillation signal (the output of the voltage-controlled oscillator 16) and the carrier signal contained in the input IF signal. In the above description, it is assumed that the output of the differential amplifier 28 can take only three values, +, -, and 0, but in practice, the output is a continuous value as shown in FIG. 4, since it is averaged by the time constant of the differential amplifier 28.
When a radio communication line is restored from a momentary break, the frequency of the carrier at the transmitting end (the carrier contained in the received signal) and the frequency of the carrier at the receiving end (the locally oscillated signal) usually are not identical with each other; as a result, the baseband signal rotates about the origin in the phase plane in FIG. 3. If this is plotted in FIG. 4, it is seen that the phase error, starting at 0, cycles through +10, +20, +30, +45 (same as -45), -30, -10, and 0. When this voltage is integrated in the loop filter 30, the output is near the center voltage. That is, when there is a frequency difference, the VCO 16 oscillates at the center frequency. This means that once the oscillation frequency of the VCO 16 is unlocked from the reference carrier frequency at the transmitting end, phase lock can never be restored.
To cope with this situation, the sweeper 40 shown in FIG. 1 was used in the prior art. For example, synchronization is detected using a frame synchronizing circuit at a subsequent stage, and if an out-of-synchronization condition due to a momentary break or other cause is detected, the sweeper 40 is activated and the frequency of the VCO 16 is slowly swept over the entire capture range. When frequency synchronization is achieved during sweeping, the sweeper 40 is stopped, and the usual phase lock operation is resumed.
In the prior art carrier regeneration circuit using such a sweeper, the time required to synchronize after the line is restored from a momentary break, is dependent on the sweeper speed. Usually, a sweeper operates with a cycle of several hundred milliseconds to several seconds. Therefore, it takes at least several hundred milliseconds to restore synchronization.